Although gram negative rod bacteremia has been a frequent and fatal complication of the medical management of the immunologically " compromised host," there have been few prospective evaluations of serum immunoglobulins, opsonizing antibodies, bactericidal antibodies, and polymorphonuclear leukocyte function association with onset of bacteremia and the subsequent clinical events leading to death or survival. This study proposes that at the UCLA Center for the Health Sciences an additional tube of heparinized blood will be drawn each time a blood culture is obtained. Polymorphonuclear leukocytes and plasma will thus be available on every patient with documented gram negative bacillemia. The immunologic parameters mentioned previously can be assessed in relation to the clinical course and changes in serum factors (particularly antibody and complement) can be measured during recovery from infection. Freshly drawn plasma will serve as a source of leukocytes whose function can be assessed by the nitroblue terrazolium dye reduction test. Whereas the latter appears useful in diagnosing bacterial infection, more information on the NBT test may be obtained on patients with leukopenia, abnormal leukocytes (e.g. acute and chronic leukemia), fungal infections, and febrile patients receiving cytotoxic or immunosuppressive therapy. This study may yield conclusive information on the incidence of gram-negative bacilemia in the presence of opsonizing antibodies, bactericidal antibodies, normal polymorphonuclear leukocyte function, and whether multiple impairments in such host factors are necessary for bacillemia to occur, persist, and cause the death of the host.